The present invention relates to capacitance sensing circuits and, more particularly, to mutual capacitance sensing circuits for capacitive touch-pad interfaces.
A conventional capacitive touch pad has one or more transmit electrodes and one or more receive electrodes, where each transmit electrode is separated from a corresponding receive electrode by a non-conductive gap. Electronic circuitry intermittently measures the mutual capacitance between the pairs of transmit and receive electrodes. When an object such as a finger is moved close to a transmit and receive electrode pair—referred to herein as a touch, the mutual capacitance between the transmit and receive electrodes is altered and this change can be detected by the electronic circuitry, which may consequently determine, if the change in capacitance exceeds some threshold, that a touch has occurred. The conventional circuitry detects the above-described changes in mutual capacitance by measuring a corresponding voltage and using a linear voltage-to-current converter to feed a current integrator whose parameters are monitored to determine a mutual capacitance.
For example, the integrator may include an integrating capacitor, a comparator, and a counter, where the counter counts the number of clock cycles while the voltage of the integrating capacitor rises from a ground or common voltage to a reference voltage, thereby providing an indication of the mutual capacitance between the transmit and receive electrodes. The count number is then compared against a threshold value to determine whether a touch has occurred.
It is advantageous to have sensitive mutual capacitance detection circuitry.